mmc: sdhci: Fix parameter of sdhci_do_start_signal_voltage_switch()

3714f4315354 ("mmc: sdhci: update signal voltage switch code") changed the
type of the second parameter of sdhci_do_start_signal_voltage_switch(),
from "struct mmc_ios *ios" to "int signal_voltage" which causes the
following build warning:

drivers/mmc/host/sdhci.c:2044:2: warning: initialization from incompatible pointer type [enabled by default]
drivers/mmc/host/sdhci.c:2044:2: warning: (near initialization for 'sdhci_ops.start_signal_voltage_switch') [enabled by default]

Use the previous type so that it matches the start_signal_voltage_switch()
definition from host.h.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Reviewed-by: Johan Rudholm <johan.rudholm@stericsson.com>
Signed-off-by: Chris Ball <cjb@laptop.org>